  plwind(xmin, xmax, ymin, ymax)
  ******************************

  plwind

  Sets up the world coordinates of the edges of the viewport.

  xmin (PLFLT, input): The world x coordinate of the left-hand edge of the
  viewport.

  xmax (PLFLT, input): The world x coordinate of the right-hand edge of
  the viewport.

  ymin (PLFLT, input): The world y coordinate of the bottom edge of the
  viewport.

  ymax (PLFLT, input): The world y coordinate of the top edge of the
  viewport.

